1. Field of Invention
This invention relates to the field of electrographic printing, also known as electrography. In electrographic printing, an invisible electrical charge image is deposited onto a receiving surface. This surface is then placed in contact with "toner" comprising a large number of small, colored particles. The toner adheres to the receiving surface in charged regions, and does not adhere to the surface in un-charged regions, thus rendering the electrical charge image visible. In particular, the instant invention teaches a method and apparatus for cleaning and lapping the surface of the writing head in an electrographic printer.
2. Prior-Art
Electrographic printers are manufactured and sold by Xerox ColorgrafX Systems, Inc., 5853 Rue Ferrari, San Jose, Calif. 95138 U.S.A., and others. These printers typically comprise a supply roll of electrographic medium, one or more electrographic writing heads, one or more "developing stations," a drive roller for moving the medium, and a take-up roller for spooling the medium after it is printed. The writing head deposits an electrical charge image on the medium, and the developing station applies "toner" to make the image visible.
The deposition and development of electrographic images by printers of the above type is well-understood by those skilled in the art of electrographic printing. These aspects of the electrographic printing process will not be described further except as they apply to the present invention.
Two modalities are commonly applied in electrographic printing of color images: multi-pass and single-pass printing.
Background--Prior-Art--Multi-Pass Printing--FIG. 1
A multi-pass printer 5 of the type sold by Xerox ColorgrafX Systems is shown schematically in FIG. 1. Printer 5 acts under instructions provided from its own internal memory (not shown) or from memory and programs stored in external computer 7. Medium 10 (electrographic paper, film, and the like) on which an image is to be printed is supplied on supply roll 20. In normal operation, medium 10 is pulled by drive roller 30 across writing head 40. Writing head 40, generally at a fixed distance from roller 30, deposits (or "writes") an electrostatic charge pattern generally comprising dots on the surface of medium 10 as medium 10 passes over the top surface of head 40. A "back electrode" 45 (discussed infra) springably forces medium 10 into intimate contact with head (40). In the case of front-writing printers (discussed infra) a pressure pad is used in lieu of electrode 45.
A restraining torque is applied to supply roll 20 by a motor (not shown). This torque maintains tension in medium 10 in the region between drive roller 30 and supply roll 20. This tension prevents wrinkling of medium 10 in this region. Similarly, an advancing torque is applied to take-up roller 50 by a motor (not shown). The tension from this torque prevents wrinkling of medium 10 in the region between drive roller 30 and take-up roller 50. The motion and position of any given location on medium 10 relative to rolls 20 and 50 is determined by the rotation of drive roller 30.
The electrostatic image on medium 10 is "developed" in well-known fashion by passing over a "toner" bath, provided by one of toner "fountains" 60, 70, 80, or 90. Toners comprise a slurry of sub-micron sized, electrically-charged, colored particles. Although four fountains are shown, electrographic printers may employ fewer or more than four, depending on the number of colors to be printed. In this method of printing, the primary colors cyan, magenta, yellow, and black are generally used. These colors are applied sequentially in "passes", normally in the order black-cyan-magenta-yellow.
Fountain 80 is shown in a raised position. It applies the black toner to the surface of medium 10. Black toner is generally supplied to fountain 80 by a pump (not shown). The toner is drawn from a reservoir (not shown), passes through fountain 80, and returns to the reservoir. The electrostatic image containing the pictorial information for the black printing pass comprises regions of one electrostatic charge (positive or negative). Toner particles of the opposite electrical charge adhere to image areas on medium 10 in proportion to the amount of charge present, in well-known fashion. After the first pass (typically black) has been printed and the image is rolled up onto take-up roll 50, the motion of medium 10 is stopped, then reversed. Medium 10 is re-wound onto supply roll 20 to a point preceding the start of the first image. Then the next color pass is printed. This process is repeated until all the desired color passes have been printed. During the printing process, internal components of the multi-pass printer become contaminated by accretions of dust, toner residue, print medium residue, and the like. These contaminants eventually reach a level beyond which print quality suffers. At this point, the printer must be opened and thoroughly cleaned. Revenue is lost during this printer "down time."
Prior-Art--Single-Pass Printing--FIG. 2
A single-pass printer 6 of the type sold by 3M Company, Minneapolis, MN, USA is shown schematically in FIG. 2. Printer 6 acts under instructions provided from its own internal memory (not shown) or from memory and programs stored in external computer 7. Medium 10 from supply roll 20 passes sequentially over a first electrostatic writing head 110, and a first toning fountain 120. This first writing and toning activity typically comprises the printing of the black primary color image. Medium 10 continues moving away from roll 20 and passes sequentially through writing and toning stations 130 through 180, respectively. Back electrodes or pressure pads (115) springably force medium 10 into intimate contact with the writing heads. Medium 20 passes over drive roller 30 on its way to take-up roll 50. In this printer configuration, the motion of medium 10 is typically, though not always, continuous and unidirectional. As with multi-pass printers, the internal components of single-pass printers also become contaminated with print medium residue, toner residue, dust, and the like. The printer must be stopped, opened, and thoroughly cleaned before printing can resume.
Prior-Art--Charge Deposition--FIGS. 3-8
In the prior-art configurations, the latent (undeveloped) electrostatic image is deposited on the top (receiving) surface of medium 10 by minute, electrostatic discharges at the surface of the medium. Two charge deposition or "writing" methods are typically used in the prior-art electrographic printer configurations described supra. The first method is known as "back writing." The second method is known as "front writing." The two methods are distinguished by their respective electrode configurations. Both cause equivalent electrical charges to be deposited on the surface of the medium. Therefore only back writing will be discussed in detail here. Refer to FIG. 3. A relatively large back electrode 300 is in contact with the back side of medium 10. Electrode 300 typically has a dimension of 1.0 inch (2.54 cm) in the medium motion (or "process") direction. The extent of electrode 300 in the transverse direction varies between about 0.5 inch (1 cm) and 54 inches (137 cm), depending on another configurational variant in the design of electrographic printers (not discussed here). A writing or "front" electrode 340 is typically made of a metal such as copper or nickel. Electrode 340 is typically supplied as a wire having a diameter of 0.003 inch (0.076 mm). Alternatively a printed circuit trace having cross-sectional dimensions of 0.0025 inch (0.064 mm) by 0.001 inch (0.025 mm) is used.
Medium 10 typically comprises at least two layers. A back layer 320 is typically 0.005 inch (0.13 mm) thick. It is made electrically conductive by the incorporation of certain additives (not discussed here). A front layer 330 is typically 0.0002 inch (5 microns) thick. Front layer 330 is an insulating, plastic material. Layer 330 is not soluble in the liquid toner which is applied by fountains 60-90 (FIG. 1) and 110-180 (FIG. 2).
When a potential difference on the order of 500 to 1,000 volts is applied between electrodes 300 and 340, an electrical discharge occurs beneath and in the vicinity of electrode 340. The discharge occurs preferentially at electrode 340 and not at electrode 300 for two reasons. The first reason is due to the difference in size between the two electrodes. The electric field gradient in the vicinity of the smaller electrode 340 is higher because of the difference in size between the two electrodes.
The second reason relates to the quality of the physical contact between electrodes 300 and 340 and their respective mating surfaces at the back side of back layer 320 and front side of front layer 330, respectively. Refer to FIG. 4. Back electrode 300 and back layer 320 are in intimate contact over a large area. The low electrical impedance associated with this large intimate contact area minimizes the possibility of electrical discharge on the back side of medium 10 when the writing voltage is applied. In contrast, front electrode 340 and front surface 330 are maintained in less-than-intimate contact by the presence of abrasive, pigment particles 400, typically titanium dioxide which are incorporated into layer 330 at the time of manufacture of medium 10.
Front surface 330 is typically 5 microns thick. Pigment particles 400 are typically 6 or 7 microns in diameter and thus project beyond the surface of layer 330 by a distance of approximately two microns. Particles 400 serve three purposes. The first is to add whitening to medium 10. The second is to continually abrade and clean the surface of electrode 340. The third purpose of pigment particles 400 is to provide a high-impedance air gap 410 between the external surface of insulating layer 330 and electrode 340.
The electrical discharge which occurs, preferably at gap 410, leaves image-wise, electrostatic charges on the surface of medium 10. This discharge occurs at a voltage value determined by Paschen's Law. According to Paschen's Law, the voltage at which a spark or discharge will occur between two parallel plate electrodes varies according to the plot shown in FIG. 5. The ordinate is the voltage at which discharge occurs. The abscissa is the product of pressure, p (mm Hg), and the distance between the electrodes, d (cm). In the present case, the exterior surface of layer 330 (FIG. 4) and the top of electrode 340 (FIG. 4) comprise the two electrodes. These electrodes are held apart by pigment particles 400 (FIG. 4) at a spacing of about two microns. At two microns and standard atmospheric pressure (760 mm Hg), the discharge voltage is approximately 500 volts. It was mentioned supra that layer 330 is an insulator. Yet it has also been regarded an electrode in the present discussion, which implies that it is a conductor. This apparent contradiction is resolved by the transient nature of the voltage applied to electrodes 300 and 340. In order to deposit image-wise charge dots as medium 10 moves, the writing voltage must be applied in the form of transient pulses. The rise time of these pulses is sufficiently short to render the capacitive reactance of layer 330 very small. Thus, for a brief period of time most of the potential difference between electrodes 300 and 340 appears in the air gap between the surface of layer 330 and electrode 340 and a discharge will occur. Note, by reference to FIG. 5, that very small differences in the spacing between layer 330 and electrode 340 result in significant changes in the voltage at which a discharge will occur. The importance of this fact becomes apparent infra.
In a typical electrographic printer, many writing electrodes 340 are placed side-by-side as shown in FIG. 6. Such an assemblage of electrodes 340 et seq. is called a writing "head." Each electrode is connected to a voltage source and, on demand, deposits charge on the external surface of layer 330, as described supra.
A writing head is shown in detail in FIG. 7. Electrodes 340 et seq. are typically embedded in an insulating, plastic material 700. This material provides mechanical support for the individual electrodes. A typical dimension of head 700 in the process direction is 1.0 inch (2.54 cm). A typical length of head 700 in a direction transverse to the process direction is 54 inches (137 cm).
FIG. 8 shows a cross-sectional view of head 700. Medium 10 is shown in contact with head 700. Intimate contact is maintained by back electrode 300, as described supra. During the writing process, static electrical charge is deposited on the surface of medium 10 as it moves over electrodes 340. As explained supra, the deposition of electrical charge is dependent on the maintenance of minute spacings between the lower surface of medium 10 and electrodes 340 in head 700. If these spacings are not accurately maintained across the width of head 700 (transverse to the process direction), varying voltages will be required to create the discharges which leave static electrical charge on the surface on medium 10. As a result of varying discharge voltages, varying amounts of current will be available for each discharge. This results in the deposition of varying amounts of charge on the surface of medium 10. Therefore the result of varying spacings is variations in density of the final print in the cross-process direction. These are normally called "striations," and in extreme cases "dropouts." Striations and dropouts can be sufficiently objectionable as to cause rejection of printing jobs. This in turn wastes valuable time and money.
Striations and dropouts caused by varying spacings are typically the result of uneven build-up of contaminants 810 on head 700. These contaminants arise from agglomerations of dust, dried toner, dislodged pigment particles, and the like. These agglomerations must be periodically removed in order to maintain print quality. To remove these deposits, the printer is stopped, resulting in a loss of productivity. The printer is opened and head 700 is manually scrubbed using a cloth or lint-free paper and a solvent, typically a kerosene-like synthetic hydrocarbon manufactured by Exxon Corporation, of Houston, Tex., U.S.A. and sold under the trademark Isopar. The printer is then closed, medium 10 is properly re-threaded, and production resumes.
This cleaning process is usually started after at least one printed image has been deemed unsatisfactory, resulting in wastage of at least one print. The cleaning process typically results in at least 10 minutes "down time," during which the printer is inactive and not generating revenue and the operator's attention is diverted from more productive activities. In some circumstances, the printer must be cleaned after each 50 feet (18 m) have been printed. This can result in a substantial and undesirable addition to production costs.
Occasionally, the buildup of deposits will cause head 700 to wear unevenly in the cross-process direction (perpendicular to the process direction in the plane of medium 10). Uneven wear can cause variations in discharge potential, as described supra. This, in turn, results in the presence of striations and dropouts. In order to return to normal print quality, head 700 must be "lapped." Again, the printer must be stopped and opened to expose head 700. A fine, abrasive is carefully rubbed across the top surface of head 700, in the cross-process direction. Frequently a lapping tool (not shown) is used to ensure that the surface is perfectly uniform after lapping. The lapping operation can take as long as 20 minutes. Although lapping is normally done in the cross-process direction, it is possible that lapping in this direction is inappropriate. Lapping in this direction removes the minute "wear-in" differences in height which normally occur when an abrasive medium (such as electrographic paper, film, etc.) passes between two surfaces which are sprung together, i.e. back electrode 300 (FIG. 8) and head 700 (FIG. 8). Lapping in the cross-process direction removes these variations and may actually contribute to the formation of striations and dropouts.